In olefin epoxidation an olefin is reacted with oxygen to form an olefin epoxide, using a catalyst comprising a silver component, usually with one or more further elements deposited therewith on a carrier. The olefin oxide may be reacted with water, an alcohol or an amine to form a 1,2-diol, a 1,2-diol ether or an alkanolamine. Thus, 1,2-diols, 1,2-diol ethers and alkanolamines may be produced in a multi-step process comprising olefin epoxidation and converting the formed olefin oxide with water, an alcohol or an amine.
The performance of the silver containing catalyst may be assessed on the basis of selectivity, activity and stability of operation in the olefin epoxidation. The selectivity is the molar fraction of the converted olefin yielding the desired olefin oxide. As the catalyst ages, the fraction of the olefin reacted normally decreases with time and to maintain a constant level of olefin oxide production the temperature of the reaction is increased. However this adversely affects the selectivity of the conversion to the desired olefin oxide. In addition, the equipment used can tolerate temperatures only up to a certain level so that it is necessary to terminate the reaction when the reaction temperature would reach a level inappropriate for the reactor. Thus the longer the selectivity can be maintained at a high level and the epoxidation can be performed at an acceptably low reaction temperature, the longer the catalyst charge can be kept in the reactor and the more product is obtained. Quite modest improvements in the maintenance of selectivity over long periods yields huge dividends in terms of efficiency in the olefin epoxidation process and, if applicable, also in the overall process for the production of a 1,2-diol, a 1,2-diol ether or an alkanolamine.
An organic halide, for example a chlorohydrocarbon, may be added to the feed to an epoxidation reactor as a reaction modifier for increasing the selectivity. The reaction modifier suppresses the undesirable oxidation of olefin or olefin oxide to carbon dioxide and water, relative to the desired formation of olefin oxide, by a so-far unexplained mechanism.
U.S. Pat. No. 4,766,105 and U.S. Pat. No. 4,761,394 disclose that rhenium may be employed as a further element in the silver containing catalyst with the effect that the initial selectivity of the olefin epoxidation is increased. Working examples given in these US patents show a trend towards a higher selectivity at higher rhenium levels up to about 3 mmole rhenium/kg catalyst, on a carrier having a surface area of 0.42 m2/g.
EP-A-352850 teaches that the then newly developed catalysts, comprising silver supported on alumina, promoted with alkali metal and rhenium have a very high selectivity. It was found that when operating with the newly developed commercial catalysts comprising silver, alkali metal promoters, and a rhenium promoter on an alumina support, longer catalyst lives are obtained when the chlorohydrocarbon level is increased over the period of operation of the catalyst, that is along with the reaction temperature increase as commonly practiced to reduce the effects of catalyst deactivation.
Not withstanding the improvements already achieved, there is a desire to further improve the performance of a rhenium containing catalyst, in particular increase the stability of operation of such catalyst.